optimise iteration over dict(something).items()

Cython/Compiler/Optimize.py

@@ -159,14 +159,25 @@ class IterationTransform(Visitor.VisitorTransform):
         if function.is_attribute and not reversed and not arg_count:
             base_obj = iterator.self or function.obj
             method = function.attribute
+            # in Py3, items() is equivalent to Py2's iteritems()
+            is_safe_iter = self.module_scope.context.language_level >= 3
+
+            if not is_safe_iter and method in ('keys', 'values', 'items'):
+                # try to reduce this to the corresponding .iter*() methods
+                if isinstance(base_obj, ExprNodes.SimpleCallNode):
+                    inner_function = base_obj.function
+                    if (inner_function.is_name and inner_function.name == 'dict'
+                            and inner_function.entry
+                            and inner_function.entry.is_builtin):
+                        # e.g. dict(something).items() => safe to use .iter*()
+                        is_safe_iter = True
 
-            is_py3 = self.module_scope.context.language_level >= 3
             keys = values = False
-            if method == 'iterkeys' or (is_py3 and method == 'keys'):
+            if method == 'iterkeys' or (is_safe_iter and method == 'keys'):
                 keys = True
-            elif method == 'itervalues' or (is_py3 and method == 'values'):
+            elif method == 'itervalues' or (is_safe_iter and method == 'values'):
                 values = True
-            elif method == 'iteritems' or (is_py3 and method == 'items'):
+            elif method == 'iteritems' or (is_safe_iter and method == 'items'):
                 keys = values = True
 
             if keys or values:

tests/run/iterdict.pyx

@@ -470,3 +470,15 @@ def optimistic_iterdict_change_size(d):
         if count > 5:
             break # safety
     return "DONE"
+
+
+@cython.test_assert_path_exists(
+    "//WhileStatNode",
+    "//WhileStatNode//DictIterationNextNode")
+def values_of_expression(**kwargs):
+    """
+    >>> sorted(values_of_expression(a=3, b=4))
+    [3, 4]
+    """
+    # this can be optimised even in Py2
+    return [ arg for arg in dict(kwargs.items()).values() ]